Type I diabetes, or IDDM, is an autoimmune disease caused by T cells that attack and destroy the insulin-producing cells located in the islets of the pancreas (Castano and Eisenbarth, 1990). The autoimmune process culminating in IDDM begins and progresses without symptoms. The disease surfaces clinically only when the cumulative loss of .beta.-cells exceeds the capacity of the residual .beta.-cells to supply insulin. Indeed, the collapse of glucose homeostasis and clinical IDDM is thought to occur only after 80-90% of the .beta.-cells have been inactivated by the immune system. Thus, patients who can be identified as suffering from IDDM are bound to be in an advanced stage of autoimmune destruction of their .beta.-cells. Moreover, diagnosis of incipient, pre-clinical diabetes by the detection of immunological markers of .beta.-cell autoimmunity can be made only after the onset of the autoimmune process. Therefore, the therapeutic quest is to find a safe, specific and effective way to turn off an autoimmune process that is already well underway.
The present inventors have examined this question before by studying the spontaneous diabetes developing in mice of the NOD strain, which is considered to be a faithful model of human IDDM (Castano and Eisenbarth, 1990). The spontaneous autoimmune process resulting in diabetes in the NOD mouse is first detectable as a mild insulitis beginning at about 1 month of age. In most female mice, the insulitis progresses towards a penetrating intra-islet infiltrate that leads to .beta.-cell damage and overt insulin-dependent diabetes mellitus (IDDM) that surfaces at about 4-5 months of age, (Bach, 1991). The insulitis is associated with T-cell autoreactivity and autoantibodies to a variety of self-antigens (Harrison, 1992). It has been assumed that one of the autoantigens is the primary target for an insulitis which could activate autoimmunity to additional, secondary self-antigens. The T-cell response to glutamic acid decarboxylase (GAD) was reported to be detectable before the T-cell responses to hsp60 and to other antigens (Kaufman et al., 1993; Tisch et al., 1993) and the administration of GAD by intrathymic injection at 3 days of age (Kaufman et al., 1993), or by intravenous injection at 3 weeks of age (Tisch et al., 1993), was found to inhibit the development of T-cell reactivity to the other self-antigens, including hsp60, and to prevent diabetes. The investigators concluded that autoimmunity to GAD might be the primary event leading to diabetes. However, a variety of seemingly "non-specific" manipulations applied early in the course of insulitis can prevent or delay the later development of diabetes (Bowman et al., 1994).
In addition to the GAD antigen, autoimmunity to hsp60 has been shown to have a functional role in NOD diabetes (PCT Patent Publication No. WO 90/10449): T cells responsive to the hsp60 peptide p277 could adoptively transfer diabetes or, when attenuated, could vaccinate mice against diabetes (Elias et al., 1991); and a single, subcutaneous administration of peptide p277 in oil either early (Elias et al., 1991) or very late in the autoimmune process (Elias et al., 1994) could arrest the disease.
T cells of the CD4 "helper" type have been divided into two groups by the characteristic cytokines they secrete when activated (Mosmann et al., 1989). TH1 cells secrete IL-2, which induces T-cell proliferation, and cytokines such as IFN.gamma., which mediate tissue inflammation; TH2 cells, in contrast, secrete IL-4 and IL-10. IL-4 helps .beta.-cells secrete antibodies of certain IgG isotypes and suppresses the production of TH1 inflammatory cytokines (Banchereau et al., 1994). IL-10 indirectly inhibits TH1 activation by affecting antigen-presentation and inflammatory cytokine production by macrophages (Moore et al., 1993).
It has now been discovered by the laboratory of the present inventors that the successful treatment of the autoimmune process in IDDM by administration of the peptide p277 in oil is caused by the effect of this treatment in aborting TH1-type autoimmunity to several different antigens and instead activating p277 autoimmunity into a TH2 mode. Thus, a disease with a spectrum of autoreactivities can be turned off with a single peptide capable of inducing a T cell cytokine shift.